Conventional multi-beam communications satellites (e.g., spot beam satellites) and high altitude platforms (HAPs) are generally designed in a manner whereby a given geographic coverage area is serviced by a pattern of beams defined based on the geometry of the antenna elements (e.g., phased array elements) and the beam coefficients applied to the antenna. In such conventional designs, the satellite/HAP antennae generate a configuration of fixed beams radiated by the antennae, where the fixed beams are preconfigured on the satellite/HAP and sweep across the ground as the satellite/HAP moves though its orbit or flight pattern. A user terminal is thus served sequentially by the different beams as the fixed beams sweep across the geographic site or cell where the terminal is located. For example, in a typical low earth orbit (LEO) satellite system, the period of a satellite orbit is determined by the altitude of the orbit. For a satellite at about 1200 km altitude the orbital period is less than two hours. Further, in the case where the satellite employs narrow beams for high gain, the user terminal experiences a beam handover as often as every 10 to 20 seconds, where each handover requires a change in frequency and polarization on the part of the terminal, which has to be synchronized at the satellite.
Further, for example, in an non-geostationary orbit (NGSO) satellite data communications system, such as a multi-satellite LEO system, in order to provide data communications services between two ground terminals situated a relatively large geographic distance from each other, the data packets for such communications must be routed from the source satellites servicing the cell within which the source terminal is located (for the duration of the data communications) to the respective destination satellites servicing the cell within which the destination terminal is located (for the duration of the data communications). As presented below, depending on the extent of the geographic diversity of the two terminals, such data communications will require differing levels of route complexity, potentially involving a number of satellites and a number of gateways, in order for the respective data packets to traverse a wide geographic distance between the source and destination terminals. The extent of the routing complexity would depend on the number of satellites and gateways required to transmit the packets of the data communications from the source terminal to the destination terminal.
For example, assuming a relatively short distance and assuming that a single ground gateway terminal services both the source and destination satellites, the data communications may only require transferring the respective data packets from the source satellites to the destination satellites via two hops, through the ground gateway servicing the source and destination satellites, while they are servicing the respective source and destination terminals (as would be understood, in view of the transitory nature of a LEO satellite system, at different points in time a different source satellite will be servicing the source terminal and a different destination satellite will be servicing the destination terminal). Alternatively, assuming a relatively large distance between the source and destination terminals, the routing of the respective data communications packets may be required to traverse a combination of a number of satellites and a number of gateways in order to traverse such a wide geographic distance. For example, at a given point in time, the data packets may have to traverse a number of satellite-gateway-satellite hops to traverse the geographic distance. As such, in view of the transitory nature of such a system, in order to maintain a long-term or relatively permanent data communications circuit between two such terminals, the route processing overhead become significant, and also the multiple satellite-gateway hops introduce significant delay in the packet transmissions between the two terminal endpoints of the circuit.
What is needed, therefore, are approaches for efficient radio resource management and routing functions to support long-term or relatively permanent data communications circuits between two geographically diverse endpoint terminals in a non-geostationary orbit (NGSO) wireless data communications system, such as a low earth orbit (LEO) satellite system.
Some Example Embodiments
The present invention advantageously addresses the foregoing requirements and needs, as well as others, by providing a system and methods for efficient radio resource management and routing functions to support long-term or relatively permanent data communications circuits between two geographically diverse endpoint terminals in a non-geostationary orbit (NGSO) wireless data communications system, such as a low earth orbit (LEO) satellite system.
In accordance with example embodiments, a method is provided for determining a plurality of routes for a committed bit rate data communications circuit between a source ground terminal and a destination ground terminal. The circuit is formed via a plurality of satellites, each traveling in a respective non-geostationary orbit around the Earth, and a total time during which the circuit is active is divided into a plurality of time intervals of equal duration. The determination of the plurality of routes comprises (i) selecting, for each time interval, an uplink satellite of the constellation to service the source ground terminal as an ingress node for the circuit, and (ii) determining, for each time interval, a series of inter-satellite links connecting the uplink satellite selected for the respective time interval to a downlink satellite of the constellation servicing the destination ground terminal during the respective time interval, wherein the series of inter-satellite links is determined so that it will be valid for connection of the circuit between the uplink ground terminal and the downlink ground terminal during the respective time interval. the duration of each interval of the plurality of time intervals is based on a time of validity, based on the traveling of the plurality of satellites in their respective orbits, for a one of the inter-satellite links of all of the plurality of routes that is of the shortest duration.
In accordance with further example embodiments, a system for providing one or more committed bit rate data communications circuits, each between a respective source ground terminal and a respective destination ground terminal is provided. Each circuit is formed via a plurality of satellites, each traveling in a respective non-geostationary orbit around the Earth, and a total time during which the circuit is active is divided into a plurality of time intervals of equal duration. The determination of the plurality of routes comprises (i) selecting, for each time interval, an uplink satellite of the constellation to service the source ground terminal as an ingress node for the circuit, and (ii) determining, for each time interval, a series of inter-satellite links connecting the uplink satellite selected for the respective time interval to a downlink satellite of the constellation servicing the destination ground terminal during the respective time interval, wherein the series of inter-satellite links is determined so that it will be valid for connection of the circuit between the uplink ground terminal and the downlink ground terminal during the respective time interval. the duration of each interval of the plurality of time intervals is based on a time of validity, based on the traveling of the plurality of satellites in their respective orbits, for a one of the inter-satellite links of all of the plurality of routes that is of the shortest duration.
According to further example embodiments, the antennae of each satellite may be configured to form communications beams to illuminate a fixed/stationary pattern of cells on the ground as the satellite travels through its orbit. According to one such embodiment, the satellite is configured to generate rapidly time varying beams, with different scan angles, beam shapes and directivities, in order to illuminate the respective fixed/stationary ground-based cell pattern. While the beam/footprint of a satellite moves across the surface of the Earth relative to the movement of the satellite, the cells remain stationary or fixed (each satellite antenna continually adjusts the relative cell pattern of its respective beam to maintain a fixed/stationary cell pattern on the surface of the Earth—as the satellite moves, the satellite will continually service the cells of that cell pattern that are within the footprint of the satellite antenna beam). By way of example, a phased array antenna is employed on satellite, and the satellite is configured to dynamically determine the appropriate beamforming coefficients or weights, over given time intervals, in order to generate time varying beam patterns that illuminate the respective stationary cells on the ground. Further, each such ground-based cell may be assigned a fixed frequency/polarization (F/P) pair. Accordingly, as the communications platform moves, it forms the communications beams to illuminate the respective stationary cell pattern, where each cell is assigned a respective F/P pair. As such, a ground terminal located within a given cell utilizes a constant F/P pair for the fixed cell within which it is located irrespective of the satellite that is servicing the cell.
Still other aspects, features, and advantages of the present invention are readily apparent from the following detailed description, simply by illustrating a number of particular embodiments and implementations, including the best mode contemplated for carrying out the present invention. The present invention is also capable of other and different embodiments, and its several details can be modified in various obvious respects, all without departing from the spirit and scope of the present invention. Accordingly, the drawing and description are to be regarded as illustrative in nature, and not as restrictive.